Buying at flea markets to sell.

If you register as self-employed (whether or not in secondary occupation) without a business form (such as BV, VOF, CV, NV, VZW, etc.), you remain an economic (and fiscal) entity. You can then sell all the junk you currently own (you must record receipts in a diary, which you must also fill in on days when you do not make any sales; receipts via bank transfer, QR payments, Bancontact or credit cards must be recorded either in the journal or on the invoice, and sales to other companies must be invoiced (because the vAT authorities need to know who pays vAT to whom).
You can do your purchases from other company (many sellers at a flea market are also self-employed) against a receipt or invoice. Purchases you do from private persons should also be recorded in your diary if you pay them in cash, or you question them to sign a receipt (you must have a document to be able to record them as expenses) or to provide an invoice. For larger purchases, it is best to draw up a sales agreement describing what you have purchased, that it is second-hand, on what date and for what amount. Also include your vAT number on that document.
Johan

Thank you, Johan. This is clearer to me now. I just don't understand whether I should refuse the purchase if the seller is a private person and does not want to provide personal details or sign anything.